Impact-resistant heterophasic propylene polymer compositions are well known in the art. Typically, they comprise a relatively high crystalline propylene polymer fraction mostly insoluble in xylene at 25° C., and a relatively low crystallinity copolymer fraction being highly soluble in xylene at 25° C. The relatively high crystallinity fraction is generally a propylene homopolymer, or a random propylene copolymer with a relatively low amount of olefin comonomer, characterized by high isotacticity. The relatively low crystallinity fraction is generally a propylene copolymer and particularly a propylene-ethylene copolymer having a content of ethylene ranging from 15 to 75% by weight. Those heterophasic compositions can be prepared by several methods, including the mechanical blend of the two components. The preferred method, however, is the preparation in-reactor by a sequence of polymerization step carried out in one or two reactors. Usually, in a first step propylene is polymerized alone or copolymerized with a small amount of other olefins in order to produce the high crystallinity fraction, while in a second step carried out under different polymerization conditions, in the presence of relatively high amount of other olefin comonomer, the xylene-soluble fraction is produced.
This method is largely used industrially and it is preferably carried out by operating in two different reactors which can be based on the same or different polymerization technology. In particular, the first step can be carried out in a liquid phase reactor or in gas-phase reactor, while the second step is commonly carried out in gas-phase in order to avoid the dissolution of the low crystallinity fraction in the reaction bath.
In this type of process the performances of the catalyst is very important. The catalyst system should be able to produce both a highly isotactic propylene (co)polymer in the first step, and in the second step a copolymer in which the comonomer units are sufficiently well distributed along and among the polymer chains in order for the resulting copolymer to have a low crystallinity, i.e. high solubility in xylene, which confers impact resistance to the composition. Of course, it is simultaneously requested a high polymerization activity in order to maintain at an acceptable level the plant productivity. Due to the presence of multiple polymerization step and to the fact that a certain weight balance among the two polymer fractions must be kept, the catalyst needs to maintain an acceptable level of polymerization activity over the time and in particular it should be able to maintain the necessary level of reactivity in gas-phase. Moreover, the catalyst should have the necessary morphological versatility to withstand the initial stage of polymerization where the crystalline polymer is produced while at the same time maintaining the capability to prevent that in a successive step the soluble polymer fraction exits the polymer/catalyst growing granule and adheres to the reactor.
The foregoing explanations make it clear that the required catalyst system is requested of performances and versatility which are quite demanding and difficult to be found in a single catalyst. In fact, WO2003/054035 teaches to use a combination of two different catalysts in order to have simultaneously high productivity and sufficient porosity for the preparation of the soluble polymer fraction. The use of catalyst mixtures, however, introduces some complexity into the catalyst-handling section of the plant which would require more devices in order to correctly use them. Moreover, as each single catalyst of the mixture is produced by a distinct batch run, the likelihood to have variations on the final catalyst is doubled and so is the likelihood to have of a polymer composition out of specification. Moreover, due to the presence of different polymerization stages under different conditions, the behavior of a combination of catalysts is hardly predictable. In fact, each catalyst may have a different behavior under certain polymerization conditions and the specific results would need to be checked. A mixture of catalysts may, for example, have an excellent activity but poor behavior in terms of xylene-soluble polymer material incorporation.
Accordingly, documents that only generically disclose suitability of catalysts or mixture of catalysts for preparation of impact propylene copolymers do not actually convey any concrete teaching in the absence of working examples. This is the case for example of WO2007/147864 and WO2007/147865. The first document suggests using a blend of two Ziegler-Natta catalysts containing two different donors, a succinate and a diether respectively. The second teaches to use a single catalyst containing a blend of the said two donors. In both cases the target was the obtainment of a propylene polymer product having characteristics intermediate among those of the products obtained by the use of the catalysts based on the individual donor. Said documents mention very generally, without any concrete example, that the proposed solution could be suitable for production of propylene heterophasic copolymers. They do not offer any concrete indications on how to prepare the catalysts.
In WO2010/146074 and WO2011/061134 the inventors set out to provide improved processes for preparing heterophasic polypropylene compositions by employing catalyst systems based on a solid catalyst component having average particle size ranging from 15 to 80 μm comprising a magnesium halide, a titanium compound having at least a Ti-halogen bond and at least two electron donor compounds one of which being present in an amount from 50 to 90% by mol with respect to the total amount of donors and selected from succinates and the other being selected from 1,3-diethers. Propylene polymer compositions obtained according to both those documents are said to be characterized by an excellent impact resistance/rigidity balance.
There is however still room for improvement in processes for the preparation of impact resistant polypropylene compositions, particularly by providing a high efficiency process able to deliver compositions with further improved impact properties.